Langmuir 1992,8, 357-359
357
Toward Orthogonal Self-Assembly of Redox Active Molecules on Pt and Au: Selective Reaction of Disulfide with Au and Isocyanide with Pt James J. Hickman,t Paul E. Laibinis,* David I. Auerbach,t Chaofeng ZOU,~ Timothy J. Gardner,? George M. Whitesides,$ and Mark S. Wrighton’pt Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, and Department of Chemistry, Harvard University, Cambridge, Massachusetts 02138 Received November 15, 1991 Surfacesof platinum and gold adsorb 12-ferrocenyldodecylisocyanide,I,and bis[l0-(ferrocenylcarbony1)decyll disulfide, 11, and yield self-assembled, redox active monolayers incorporating ferrocene groups. The values of Elj2 for alkylferrocenyl centers from I are -300 mV more negative than acylferrocenyl centers from 11. This difference allows easy electrochemical measurement of surface concentrations of ferrocene centers from I and I1 on Pt or Au. I adsorbs selectively on Pt or Au compared to 11, but with different degrees of selectivity. Further, for Pt the selectivity depends on the surface pretreatment. The affinity of I1 for a Pt surface first exposed to an 0 2 plasma (oxidized Pt) is lower than that for a Pt surface exposed first to an 0 2 plasma and then to a H2 plasma (reduced Pt), but the two plasma pretreatments of Pt do not affect the binding of I on this metal. The same two plasma pretreatments of Au do not influence the selectivity for binding I vs 11; the selectivity for binding I vs I1 on Au is -8:l. By use of the 0 2 plasma pretreatment and a 401 solution of 111,self-assembly of I1 onto Au and I onto Pt can be effected when the two samples are derivatized in the same solution. We wish to report procedures for formation of selfassembled monolayers that modify Au and Pt electrodes selectively when both are immersed in a common solution containing the isocyanide, I, and the disulfide, 11, eq 1. Much work has been reported recently concerning the use of self-assembly methods to functionalize electrode surfaces.’-ll Relatively little work, however, has been reported concerning the self-assembly of multicomponent systems on different substrates. We have named selec-
* Author t o whom correspondence should be addressed at M.I.T. + M.I.T.
t Harvard. (1) Swalen, J. D.;A h a , D.L.; Andrade, J. D.;Chandross, E. A,; Ga-
roff,S.;Israelachvili,L;McCarthy,T. J.;Murray,R.;Pease,R.F.;Rabolt, J. F.; Wynne, K. J.; Yu, H. Langmuir 1987, 3, 932. (2) (a) Polymeropoulos, E. E.; Sagiv, J. J.Chem. Phys. 1978,69,1836. (b) Sagiv, J. Isr. J. Chem. 1979, 18, 346. (3) (a) Nuzzo,R. G.; Allara, D.L. J.Am. Chem. SOC. 1983,105,4481. (b) Nuzzo, R. G.; Fusco, F. A.; Allara, D.L. J. Am. Chem. SOC. 1987,109, 2358. (c) Porter, M. D.;Bright, T. B.; Allara, D.L.; Chidsey, C. E. D.J. Am. Chem. SOC. 1987,109, 3559. (4) Cotton, T. M.; Kim, J. H.; Uphaus, R. A. Microchem. J. 1990,42, 44, and references therein on viologen monolayer electrochemistry.
(5) (a) Li, T. T.-T.; Weaver, M. J.Am. Chem. SOC. 1984,106,6107. (b) 1984,106, 1233. Li, T. T.-T.; Weaver, M. J. Am. Chem. SOC. (6) (a) Bain, C. D.;Whitesides, G. M. Science 1988,240,62. (b) Bain, C. D.;Whitesides, G. M. J. Am. Chem. SOC. 1988, 110, 3665, 6561. (c) Bain, C. D.;Whitesides, G. M. Agnew. Chem. 1989,101, 522. (d) Whitesides, G. M.; Laibinis, P. E. Langmuir 1990, 6, 87. (7) (a) Chidsey, C. E. D.;Loiacono, D.N.Langmuir 1990,6,682. (b) Chidsey, C. E. D.; Bertozzi, C. R.; Putvinski, T. M.; Mujsce, A. M. J. Am. Chem. SOC. 1990, 112,4301. (8)(a) Murray, R. W. In Electroanalytical chemistry; Bard, A. J., Ed.; Dekker: New York, 1984; Vol. 13, p 191, and references therein. (b) Hubbard, A. T. Chem. Rev. 1988,88, 633. (9) (a) Tsou, Y. M.; Liu, H. Y.; Bard, A. J. J.Electrochem. SOC. 1988, 135, 1669. (b) Facci, J. S. Langmuir 1987,3,525. (10) Widrig, C. A.; Porter, M. D.;Ryan, M. D.; Strein, T. G.; Ewing, A. G. Anal. Chem. 1990,62, lR, and references cited therein on derivatized electrodes. (11) Hickman, J. J.; Zou, C.; Ofer, D.;Harvey, P. D.;Wrighton, M. S.; Laibinis, P. E.; Bain, C. D.; Whitesides, G. M. J. Am. Chem. SOC.1989, 111, 7271.
tivity of the sort represented in eq 1 “orthogonal selfassemblyn,12meaning that each molecule adsorbs selectively to a different surface. We have previously demonstrated such orthogonal self-assembly for Au and A1203: thiols selectively bind to Au, and carboxylic acids bind selectively to A 1 ~ 0 3 .Part ~ ~ of the significance of our new findings is that both Pt and Au are useful electrode materials, whereas insulating A1203 is not. Thus, orthogonal self-assemblyusing I and I1can be exploited toprepare an organized two-component, redox active molecular assembly. A potentially more important application would be to take a single component molecular assembly and use the two different functional groups as “clips” to span the space between two adjacent microelectrodes. Thus, not only would selective functionalization be achieved but also orientation. Self-assembly of redox active monolayers has been accomplished by immersion of Pt and/or Au into organic solutions of 113and/or II.14J5 The Elp for the alkylferrocenyl centers from I is -300 mV more negative than Ell2 for the acylferrocenyl centers from 11, allowing easy electrochemical measurement of their surface concentra(12) Laibinis, P. E.; Hickman, J. J.; Wrighton, M. S.; Whitesidee, G. M. Science (Washington, D.C.)1989,245,845. (13) The synthesis of I will be published in the full paper detailing this work. (14) Hickman, J. J.; Ofer, D.;Zou, C.; Laibinis, P. E.; Whitesides, G. 1991,113, 1128. M.; Wrighton, M. S. J. Am. Chem. SOC. (15) Prior to derivatization the e--beam evaporated Au and/or Pt on a Si substrate was pretreated with an 02 plasma (2-5 min) or with the 02 plasma followed by a Hz plasma (1 min). Plasma treatment was done using a Harrick PDC-23G plasma cleaner. The 02 treatment was at 0.3 Torr at medium power (60 W) and the Hz treatment was at 0.3 Torr at low power (40 W). Immediately after a plasma pretreatment, the electrodes were derivatized by immersion into a deoxygenated 5 X 10-6 M solution of I or into a lo-’ M solution of I1 in 1:l EtOH/hexane solvent. Best resultafor orthogonalself-assemblywereachievedusingtheozplasma pretreatment and a 40:l solution of 1I:I in 1:l EtOH/hexane aa solvent at total concentration of adsorbate of lo-‘ M. Typical derivatization times were 18-48 h.
0 1992 American Chemical Society
Letters
358 Langmuir, Vol. 8, No.2, 1992 f
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Figure 2. Comparison of XPS data for Pt and Au pretreated with a 3-min 02 plasma or with a 3-min 02 plasma followed by a 1-min Hz plasma. Pt and Au Electrodes =ked
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spectra (XPS) in the 4f7p region, Figure 2, show high binding energy peaks consistent with the presence of a Au or Pt oxide after the 02 plasma pretreatment which is removed by the Hz plasma pretreatment.16 Ten independent comparisons show that the reduced
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Langmuir, Vol. 8, No. 2, 1992 359
Letters
Pt (02plasma followed by HZ plasma) yields about 10 times the coverage of redox active material as does the oxidized Pt ( 0 2 plasma only) when reacting both surfaces in the same solution of 11,Figure 1. Importantly, oxidized and reduced Pt derivatized with only I show the same coverage, -(4 f 1) X 10-10 mol/cm2. XPS analysis of oxidized Pt functionalized with I shows high binding energy Pt 4f7p peaks, consistent with retention of the oxidized surface. In contrast, XPS of oxidized Au after reaction with I or I1shows no detectable high binding energy peaks, consistent with loss of the Au oxide. Electrochemically formed Au oxide has also been shown to be reactive toward other surface modification reagents.17 Electrochemical analyses of pretreated Pt and Au after reaction with mixtures of I and I1 show that the isocyanide, I, is always more competitively adsorbed than the disulfide, 11,but a sufficiently large ratio of 1I:Iin solution will yield selective uptake of 11. The reduced and oxidized Au both give a preference of I vs I1 of 81. Reduced and oxidized P t , however, have different preferences for I vs 11, since oxidized Pt is nearly inert to 11. The preferences for uptake of I vs I1on oxidized and reduced Pt are 200.
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~~
(16) XPS was done using an SSL 100 spectrometer. Samples were attached to the stage via metal clips. The lowest energy Pt 4f712peak was taken to be Pt(0) and the lowest energy Au 4f7p peak was taken to be Au(O), and they were monitored as the reference to correct any peak shifta due to sample charging (Au(0) 4f7 2 is at 84.0 eV and Pt(0) 4f7p is at 71.0 eV binding energy). The X-ray iine used for excitation was the Al Ka line at 1486.6 eV. For ageneral reference,see: Handbook of X-ray Photoelectron Spectroscopy; Perkin-Elmer Corp.: Eden Prairie, MN, 1979. ~. . (17) Fischer,A. B.;Wrighton,M. S.;Umana, M.;Murray,R.W. J . Am. Chem. SOC.1979, 101, 3442.
1and -201, respectively. Thus, pretreatment of Pt and Auwith only the 0 2 plasma is predicted to give the highest degree of orthogonal self-assembly by reaction with I and I1 with Pt and Au. By using a 4 0 1 ratio of 1I:I in solution and the 02plasma pretreatment of Au and Pt, we have demonstrated selective functionalization of Pt with I and selective functionalization of Au with I1when derivatized in the same solution of I and 11. The self-assembly yields a 1:lO and 1O:l ratio of alkylferrocenyl/acylferrocenyl centers on Au and Pt, respectively, Figure 3. The total coverage of ferrocene centers is always (4 f 1)X 10-lomol/cm2,or about one monolayer. Changing the ratio of I and I1 in solution changes the ratio of the two different centers on the Au and Pt surfaces in a manner consistent with a competitive reaction of I and 11. Further studies are underway to establish the scope of orthogonal self-assembly on Pt and Au, in order to functionalize microelectrode arrays12J3with a variety of redox centers.
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Acknowledgment. We thank the Office of Naval Research and the Defense Advanced Research Projects Agency for partial support of this research. We acknowledge use of XPS and Auger facilities acquired through the joint HarvWM.1.T. University Research Initiative funded by the Defense Advanced Research Projects Agency. Support from the TRW Corp. to D.I.A. as a TRW Scholar at MIT is also gratefully acknowledged. %&try NO. I, 138180-62-8; 11,131832-59-2;Pt, 7440-06-4; Au,7440-57-5; Hz, 1333-74-0;02,7782-44-7;CHaCHzOH, 64-17-
5; n-CeHld, 110-54-3; CHaCN, 75-05-8; [n-B~N]PF6,3109-63-5.
